FIG. 1 is a highly simplified block diagram of a prior-art pulse radar system 10. Radar system 10 includes a radar antenna 12 which is connected to a port of a transmit/receive (T/R) device 14. T/R device 14 receives recurrent guided-wave electromagnetic pulses to be transmitted from a transmitter (TX) 16, and couples the pulses to be transmitted to the antenna 12. Antenna 12 transduces the electromagnetic signals into unguided waves which propagate in free space toward a target region 18, as suggested by the “lightning” bolt symbol 20. If target region 18 contains a target, such as the illustrated airplane 22, some of the energy of each of the transmitted electromagnetic pulses is reflected toward the antenna 12, as known to those skilled in the radar arts. The reflected or return signal pulses are represented in FIG. 1 by the lightning bolt representation 24.
Reflected electromagnetic signal pulses returning to antenna 12 are transduced by the antenna into guided waves, which propagate to the transmit/receive device 14, and are routed by the transmit/receive device 14 to a receiver illustrated as a block 30. Receivers may perform all manner of functions, but some of the important functions include low-noise amplification and frequency downconversion of the return signal pulses. The receiver may also digitize the return signals in preparation for signal processing. The sampling required for digitization is represented in FIG. 1 by a block 31. Whether in analog or digital form, the received return signal pulses retain the original transmitted pulse timing, with the superposition of timing changes attributable to the radial motion (closing or receding) of the target 22 relative to the radar antenna 12. The radial speed of the target may be estimated or determined by comparing the Doppler shift of the return signals relative to the transmitted signals by means which are not illustrated.
The return signals must traverse the distance between the antenna 12 and the target, and the reflected signals must again traverse the distance from the target to the antenna 12. The signal in each of these two paths is subject to attenuation, as known, as the square of the distance. The combination of the forward and return paths is therefore attenuated by the fourth power of the distance. It will be appreciated that the signal strength of the return signal pulses is often very small.
A technique which has long been used to improve the extraction of pulse energy from return signals is pulse-to-pulse integration. The return pulses from the receiver 30 and sampling block 31 of FIG. 1 are illustrated as being applied to an integration block 32. In the case of a stationary target, the return signal pulses retain the inter-pulse temporal spacing or interval of the original transmitted pulses, which in the case of regular recurrent pulse transmissions is a constant. FIG. 2a is a plot 210 of the recurrent transmitted pulses, some of which are designated 2101, 2122, 2123, . . . , with an interpulse period designated 210IP, and plot 212 of FIG. 2b shows the return pulses, some of which are designated 2121, 2122, 2123, . . . . In response to a transmitted pulse such as 2103 of FIG. 2a, the target reflects energy, which is received as a return pulse 2123 at some time t2123 after the transmission of pulse 2103. In the case of a stationary or fixed target, the received return pulses can be mutually delayed by multiples of the known inter-pulse interval, and as many pulses can be integrated as may be desired to raise the integrated return signal amplitude to a value greater than the system or clutter noise, as illustrated in FIG. 2c. In FIG. 2c, return pulse 2121 is illustrated as being delayed for twice (2×) the transmitted inter-pulse period to produce delayed pulse 2141. Return pulse 2122 is illustrated as being delayed for one inter-pulse period (1×) to produce delayed pulse 2142. Return pulse 2123 is not delayed, as indicated by the “no delay” notation in FIG. 2c. The sum of the delayed pulses 2121 and 2122, and of the undelayed pulse 2123, is illustrated as integrated pulse 216, which has an amplitude which is greater than that of any one of the constituent pulses alone. This increased amplitude may raise the integrated pulse 216 above any unavoidable noise.
The inter-pulse temporal spacing of the return pulses is not a constant in the case of a moving target, so the simple expedient of mutually delaying the return pulses by multiples of a fixed time does not result in temporal superposition of the pulses. Without superposition of the return pulses, the integration to increase the target signal amplitude may not be as effective as desired, and the signal-to-noise ratio (SNR) may not meet requirements. The lack of a signal-to-noise improvement may result in generation of integrated pulses which do not exceed the noise level, failure to detect a target, or inability to accurately estimate the target's location.
Radial motion of a target results in “range walk” of the return pulses. That is to say, that the target return pulses do not arrive at the radar antenna 12 with a timing equal to the inter-pulse spacing of the transmitted pulses. Instead, for a target with radial motion toward the radar antenna, each succeeding return pulse will arrive somewhat earlier, relative to the corresponding transmitted pulse, than the previous return pulse. The increasingly earlier relative time of arrival results from progressive reduction in the distance between the radar antenna and the target in the case of radial motion toward the radar antenna. In the case of a target radially receding from the radar antenna, successive return pulses from the target arrive progressively later, since the transmission and return distances are increasing. A method that has been used in the prior art for integration of pulses from a moving target is to perform a plurality of pulse integrations on a train of return pulses, with each pulse integration based upon the assumption or hypothesis that the target has a particular value of radial motion. FIG. 1 illustrates the application to integration block 32 of various closing speed (radial velocity) hypotheses from a block 34. Integration block 32 performs a plurality of return pulse integrations, each with a different closing speed hypothesis. It will be clear that the amplitude of the integrated pulse will be at a maximum when the hypothesized closing speed is closest to being correct. Block 36 of FIG. 1 represents selection of the greatest or largest integrated amplitude from among the results of these many integrations as being indicative of the correct closing speed hypothesis, and the integrated value of return as being representative of the target.
In the presence of clutter or strong noise, or in the case of weak target return pulses, the noise level may undesirably affect the perceived maximum value of the correctly integrated signal. In this case, the clutter or noise level may adversely affect the determination of closing speed.
An alternative or improved method is desired for determination of the closing speed of a target based on integrated return pulses.